The present invention relates to a biaxially oriented laminate polyester film. More specifically, it relates to a biaxially oriented laminate polyester film which can be self-recycled and has excellent electromagnetic conversion characteristics, winding properties and handling properties as a base film for a high-density magnetic recording medium.
A biaxially oriented polyester film typified by a polyethylene terephthalate film is used for a wide range of application, particularly as a base film for a magnetic recording medium due to its excellent physical and chemical properties.
Along with recent efforts made to increase the density and capacity of a magnetic recording medium, a more flat and thinner base film is now in demand. However, when the surface of a base film is flattened to retain excellent electromagnetic conversion characteristics, its slipperiness becomes insufficient. For instance, when the base film is rolled up, it is wrinkled or blocking occurs. As a result, the surface of the film roll becomes uneven, whereby the yield of products is reduced, or appropriate ranges of tension, contact pressure and speed for winding up the base film are narrowed, thereby making it extremely difficult to wind up the base film. When the slipperiness of the base film is low in the film processing step, the friction of the base film with a metal roll in contact with the base film increases, thereby generating chippings which cause lack of a magnetic recording signal, that is, a drop-out.
To improve the slipperiness of a polyester film, the following methods are generally employed: (i) one in which inert particles are deposited into a raw material polymer from the residual catalyst in the production process and (ii) one in which the surface of a film is made uneven by adding inert particles to the film. Generally speaking, the greater the size or content of particles in the film the more the slipperiness is improved.
As described above, the surface of a base film is desired to be as even as possible in order to improve electromagnetic conversion characteristics. When a magnetic recording medium is formed from a base film having a rough surface, the roughness of the surface of the base film appears on the surface of a magnetic layer even after the formation of the magnetic layer, thereby deteriorating electromagnetic conversion characteristics. In this case, the larger the size or content of particles contained in the base film the greater the surface roughness of the base film becomes and the more the electromagnetic conversion characteristics deteriorate.
To improve both winding properties and electromagnetic conversion characteristics which are conflicting properties, there is widely known means of producing a laminate film having an even surface on which a magnetic layer is formed for improving electromagnetic conversion characteristics and an opposite rough surface for improving slipperiness.
For the flat layer on which the magnetic layer is formed, a lubricant having a small particle diameter is used or the amount of the lubricant added is reduced to flatten the layer, whereas for the rough layer on the opposite side (running side) on which no magnetic layer is formed, a lubricant having a large particle diameter is used or the amount of the lubricant added is increased to roughen the layer.
That is, the flat layer on the coated side and the rough layer on the running side greatly differ from each other in the characteristic properties of a lubricant used, e.g., the type, particle diameter and amount of the lubricant.
In the case of a single-layer film, film waste generated in the production process of a film is recovered and formed into a chip which can be recycled for the production of the film. In the case of the above laminate film, the lubricant composition of a chip recovered from the laminate film differs from the lubricant compositions of a rough layer and a flat layer. Therefore, when the recovered chip is recycled for the production of a laminate film, the lubricant composition of a layer made from the recovered chip changes and affects the characteristic properties of the film.
It has recently been proposed to reuse a chip recovered from the laminate film for an intermediate layer portion (core layer portion) of a three-layer laminate film.
However, in this method, the intermediate layer portion must be thick enough to enable the recovery of a chip from the laminate film in addition to the recovery of a chip from the three-layer laminate film. Therefore, the three-layer laminate film must be made extremely thick. Even when a recovered chip containing a lubricant having a large particle diameter or a large amount of a lubricant is used in an intermediate portion, it influences the formation of protrusions on a surface layer portion. Therefore, the use of the chip is limited.
As described above, a magnetic recording medium having a higher density and a larger capacity and a base film having a smaller thickness have recently been desired. Therefore, the above three-layer laminate film becomes also thin and accordingly, it is substantially difficult to reuse a polymer (chip) recovered from the above laminate film in the intermediate layer of the above three-layer laminate film.
Therefore, as matters now stand, the polymer recovered from the laminate film is inevitably discarded, thereby boosting the costs of the film. Such discarded films are disposed of as industrial waste but it is becoming difficult to dispose of such films at present.
It is therefore an object of the present invention to provide a biaxially oriented laminate polyester film which can be self-recycled, has excellent winding and handling properties and is useful as a base film for a high-density magnetic recording medium having excellent electromagnetic conversion characteristics.
The other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention can be attained by a biaxially oriented laminate polyester film (may be referred to as xe2x80x9cfirst laminate film of the present inventionxe2x80x9d hereinafter) comprising a first polyester layer and a second polyester layer, wherein the first polyester layer has a thickness (tA) of 0.3 to 5 xcexcm, the second polyester layer contains an inert fine particle lubricant and has a thickness (tB) of 1.5 to 9 xcexcm, and the first polyester layer and the second polyester layer satisfy the following expressions (1) to (4):
WRa(B) greater than WRa(A) xe2x80x83xe2x80x83(1)
0.5xe2x89xa6tB/txe2x89xa60.9 xe2x80x83xe2x80x83(2)
10 less than tB/dBxe2x89xa660 xe2x80x83xe2x80x83(3)
t=3 to 10 xcexcm xe2x80x83xe2x80x83(4)
wherein WRa(A) is the center plane average roughness (nm) of the exposed surface of the first polyester layer, WRa(B) is the center plane average roughness (nm) of the exposed surface of the second polyester layer, tB is the thickness (xcexcm) of the second polyester layer, t is the sum of tA and tB, tA is the thickness (xcexcm) of the first polyester layer, and dB is the average particle diameter (xcexcm) of the inert fine particle lubricant contained in the second polyester layer.
Secondly, the above objects and advantages of the present invention are attained by a biaxially oriented laminate polyester film (may be referred to as xe2x80x9csecond laminate film of the present inventionxe2x80x9d hereinafter) comprising a first polyester layer and a second polyester layer, wherein
the first polyester layer has a thickness (tA) of 2 to 8.5 xcexcm, the second polyester layer contains an inert fine particle lubricant and has a thickness (tB) of 0.6 to 5 xcexcm, and the first polyester layer and the second polyester layer satisfy the following expressions (1) to (4xe2x80x2):
WRa(B) greater than WRa(A) xe2x80x83xe2x80x83(1)
0.15xe2x89xa6tB/t less than 0.5 xe2x80x83xe2x80x83(2xe2x80x2)
10 less than tB/dBxe2x89xa645 xe2x80x83xe2x80x83(3xe2x80x2)
t=4 to 10 xcexcm xe2x80x83xe2x80x83(4xe2x80x2)
wherein WRa(A) is the center plane average roughness (nm) of the exposed surface of the first polyester layer, WRa(B) is the center plane average roughness (nm) of the exposed surface of the second polyester layer, tB is the thickness (xcexcm) of the second polyester layer, t is the sum of tA and tB, tA is the thickness (xcexcm) of the first polyester layer, and dB is the average particle diameter (xcexcm) of the inert fine particle lubricant contained in the second polyester layer.